Stable aqueous dispersions of optically and electronically active phosphorene

ABSTRACT

Methods for the preparation of few-layer phosphorene, compositions thereof and related devices fabricated therefrom.

This application claims priority to and the benefit of application Ser.No. 62/389,537 filed Mar. 1, 2016—the entirety of which is incorporatedherein by reference.

This invention was made with government support under DMR-1505849 andDMR-1121262 awarded by the National Science Foundation andN00014-14-1-0669 awarded by the Office of Naval Research. The governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION

Few-layer phosphorene (FL-P) isolated by micromechanical exfoliation hasbeen widely studied, both fundamentally and in applications such ashigh-performance electronic and optoelectronic devices. Althoughmicromechanical exfoliation provides individual, high-quality FL-Pflakes, this technique is not scalable and is not amenable to large-areaapplications. Conventional approaches for mass production oftwo-dimensional (2D) nanomaterials involve chemical vapor deposition(CVD) and liquid phase exfoliation (LPE). Whereas CVD growth of blackphosphorus (BP) thin films is hindered by challenges with molecularprecursors and extreme growth conditions, LPE of BP has beendemonstrated and used for the large-scale deposition of thin films,following approaches for other 2D nanomaterials. Specifically, stable BPdispersions have been produced by LPE using high boiling point solventssuch as N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethylsulfoxide (DMSO), and N-cyclohexyl-2-pyrrolidone (CHP). With theseanhydrous organic solvents, chemical degradation from ambient O₂ andwater are avoided, but the exfoliation yield and flake size distributionare suboptimal. Furthermore, organic solvents have limited compatibilitywith methods such as ultracentrifugation, preventing structural finetuning and sorting. Such solvents also have high boiling points andsafety issues, presenting post-processing challenges.

SUMMARY OF THE INVENTION

In light of the foregoing, it is an object of the present invention toprovide stable aqueous dispersions of phosphorene and/or methods fortheir preparation, thereby overcoming various deficiencies andshortcomings of the prior art, including those outlined above. It willbe understood by those skilled in the art that one or more aspects ofthis invention can meet certain objectives, while one or more otheraspects can meet certain other objectives. Each objective may not applyequally, in all its respects, to every aspect of this invention. Assuch, the following objects can be viewed in the alternative withrespect to any one aspect of this invention.

It can be an object of the present invention to provide a method forproduction of two-dimensional phosphorene nanomaterials, such methods ascan be scalable and adaptable to large-area applications, without use oforganic solvents.

It can be another aspect of the present invention to provide one or moresuch methods without chemical degradation from ambient oxygen.

It can be another object of the present invention to provide one or moresuch methods compatible with ultracentrifugation together withstructural fine tuning and sorting via thickness and/or area dimension.

It can be another object of the present invention, alone or inconjunction with one or more of the preceding objectives, to provide forisolation and production of few-layer phosphorene in quantities requiredfor downstream processing and applications, including but not limited tothe fabrication of the electronic and optoelectronic devices.

Other objects, features, benefits and advantages of the presentinvention will be apparent from the summary of the invention togetherwith descriptions of certain embodiments thereof, and will be readilyapparent to those skilled in the art having knowledge of phosphoreneisolation and exfoliation techniques. Such objects, features, benefitsand advantages will be apparent from the above as taken into conjunctionwith the accompanying examples, data, figures and all reasonableinferences to be drawn therefrom.

In part, the present invention can be directed to a method of preparingfew-layer phosphorene. Such a method can comprise providing acomposition comprising a black phosphorus starting material; one or moreamphiphilic surface active components and deoxygenated water; sonicatingsuch a composition to provide a deoxygenated aqueous medium comprisingexfoliated phosphorous nanomaterials; and centrifuging such an aqueousmedium to provide a supernatant component comprising a polydispersepopulation of planar few-layer phosphorene nanomaterials comprisingmono-, bi-, and n-layer few-layer phosphorene nanosheets, wherein n canbe an integer selected from 3- about 6.

In certain embodiments, such a surface active component can be selectedfrom alkali metal alkylsulfates and combinations thereof. Withoutlimitation, such a surface active component can be sodiumdodecylsulfate. In certain other embodiments, such a surface activecomponent can be selected from bile salts and combinations thereof.Without limitation, such a surface active component can be sodiumcholate. In yet other embodiments, such a surface active component canbe selected from non-ionic triblock oxyalkylene copolymers andcombinations thereof. Without limitation, such a surface activecomponent can be a copolymer of oxyethylene and oxypropylene.

Regardless, such a method can also comprise separating the few-layerphosphorene nanomaterials into two or more separation fractionscomprising a subpopulation of few-layer phosphorene nanosheetscomprising a surface area, each such fraction as can compromisemonolayer phosphorene nanosheets, bilayer phosphorene nanosheets andtrilayer phosphorene nanosheets, or combinations thereof. In certainnon-limiting embodiments, separation can be achieved by centrifugingsuch few-layer phosphorene nanomaterials in a fluid medium comprising adensity gradient for at least one of a time and at a rotational ratesufficient to provide two or more separation fractions.

Without limitation, such a fluid medium can comprise aqueous iodixanol.As a separate consideration, such a fluid medium can comprise aplurality of aqueous iodixanol concentrations, such a density gradientas can comprise a range of concentration densities. In certain suchembodiments, such a few-layer phosphorene fraction can be within such adensity gradient at a substantially invariant point duringcentrifugation, and such an invariant point as can comprise a densityapproximating the buoyant density of such a few-layer phosphorenefraction.

Such a method can further comprise isolating at least one suchseparation fraction from such a fluid medium. In certain embodiments, atleast one of two or more such fractions can be deposited on a substrate.Without limitation, such a substrate and deposited few-layer phosphorenecan be incorporated into an electronic or an optoelectronic device.

In part, the present invention can also be directed to an alternatemethod of preparing few-layer phosphorene. Such a method can compriseproviding a composition comprising a black phosphorus starting material,one or more amphiphilic surface active components and deoxygenatedwater; sonicating such a composition to provide a deoxygenated aqueousmedium comprising exfoliated phosphorous nanomaterials; centrifugingsuch an aqueous medium to provide a supernatant component comprising apolydisperse population of planar few-layer phosphorene nanomaterialscomprising mono-, bi-, and n-layer few-layer phosphorene nanosheets,wherein n can be an integer selected from 3- about 6; and separating thefew-layer phosphorene nanomaterials into two or more separationfractions comprising a subpopulation of few-layer phosphorene nanosheetscomprising monolayer phosphorene nanosheets, bilayer phosphorenenanosheets and trilayer phosphorene nanosheets, or combinations thereof.In certain non-limiting embodiments, separation can be achieved bycentrifuging such few-layer phosphorene nanomaterials in a fluid mediumcomprising a density gradient for at least one of a time and at arotational rate sufficient to provide two or more separation fractions.Surface active components and fluid media density gradients can bediscussed above or elsewhere herein.

In part, the present invention can also be directed to a method of usinga deoxygenated aqueous medium to exfoliate black phosphorus and preparestable few-layer phosphorene nanomaterials. Such a method can compriseproviding a composition comprising a black phosphorus starting material;one or more amphiphilic surface active components and deoxygenatedwater; sonicating such a composition to provide a deoxygenated aqueousmedium comprising exfoliated phosphorous nanomaterials; and centrifugingsuch an aqueous medium to provide a supernatant component comprising apolydisperse population of planar few-layer phosphorene nanomaterialscomprising mono-, bi-, and n-layer few-layer phosphorene nanosheets,wherein n can be an integer selected from 3- about 6. Such few-layerphosphorene nanosheets can be substantially unoxidized andcharacterized, by degree of oxidative stability, by spectroscopicmethods of the sort described herein.

Such a method can also comprise separating the few-layer phosphorenenanomaterials into two or more separation fractions comprising asubpopulation of few-layer phosphorene nanosheets comprising monolayerphosphorene nanosheets, bilayer phosphorene nanosheets and trilayerphosphorene nanosheets, or combinations thereof. In certain non-limitingembodiments, separation can be achieved by centrifuging such a few-layerphosphorene nanomaterials in a fluid medium comprising a densitygradient for at least one of a time and at a rotational rate sufficientto provide two or more separation fractions. Again, surface activecomponents and fluid media density gradients can be as discussed aboveor elsewhere herein.

Accordingly, the present invention can also be directed to anoxidatively-stable few-layer phosphorene composition. Such a compositioncan comprise a few-layer phosphorene nanomaterial comprising mono-, bi-and/or n-layer phosphorene nanosheets, where n can be an integerselected from 3- about 6; deoxygenated water and one or more amphiphilicsurface active components of the sort discussed above or illustratedelsewhere herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the office upon request and paymentof the necessary fee.

FIGS. 1A-H. Digital images and spectra illustrating experimentalprocedure and characterization of FL-P nanosheets. A, Schematic of thepreparation method for FL-P aqueous dispersions. Deoxygenated water with2% w v⁻¹ SDS was prepared by ultrahigh purity Ar purging. The BP crystalwas exfoliated in a sealed container using tip ultrasonication, and thencentrifuged to remove unexfoliated BP crystals. The FL-P dispersion wassubsequently collected and ultracentrifuged to precipitate large flakes.The supernatant was finally redispersed in deoxygenated water. B, Imagesof the as-exfoliated BP solution (left) and FL-P solution (right). C,Low-magnification transmission electron microscopy (TEM) image of FL-Pnanosheets. D, High-resolution TEM image of a FL-P nanosheet. E,Selected area electron diffraction (SAED) pattern of FL-P nanosheets. F,Solution phase Raman spectrum of FL-P nanosheets. G, Visiblephotoluminescence (PL) spectrum of FL-P nanosheets using an excitationwavelength of 532 nm with a Si CCD. H, X-ray photoelectron spectroscopy(XPS) analysis of the FL-P nanosheets.

FIGS. 2A-B. Raman spectra of BP precipitants and few-layer phosphorene(FL-P). A, The liquid-phase Raman spectrum from BP precipitants showsthree modes at ˜362 cm⁻¹, ˜439 cm⁻¹, and ˜467 cm⁻¹ with full width halfmaximum (FWHM) values of ˜1.96 cm⁻¹, ˜2.12 cm⁻¹, and ˜1.95 cm⁻¹,respectively. B, Conversely, the spectrum from FL-P shows three modes at˜362 cm⁻¹, ˜439 cm⁻¹, and ˜466 cm⁻¹ with FWHM values of ˜2.96 cm⁻¹,˜3.31 cm⁻¹, and ˜5.13 cm⁻¹, respectively.

FIGS. 3A-C. XPS data for FL-P prepared with different surfactants. P 2pcore level spectra for sodium dodecylsulfate (violet, 3A), Pluronic F68(blue, 3B), and sodium cholate (green, 3C).

FIGS. 4A-J. Digital images and graphs illustrating surface propertiesand exfoliation yield of BP in aqueous solution. A, Image of a BPdispersion in deoxygenated water with and without SDS. B, Image of awater contact angle measurement on an as-exfoliated flat BP crystalsurface. C, Optical absorbance spectra of BP dispersions with (red) andwithout (blue) SDS. D, Zeta potential measurement of BP in water (blue),SDS-water (orange), and BP in SDS-water (red). E, F, Images of BPdispersions in SDS-water and NMP after sonication and centrifugation at0.5 k, 5 k, 10 k, and 15 k r.p.m. G, Concentration of the BP dispersionsfrom part (e) and (f). H, I, AFM height images of BP nanosheetsprocessed in SDS-water and NMP. J, Thickness distribution of BPnanosheets in SDS-water (red) and NMP (green).

FIG. 5. Extinction measurements of FL-P dispersions in sodiumdodecylsulfate (SDS)-water. The extinction coefficient is found to be209±2 Lg-1m-1, using the absorbance per length (A/l) at 660 nm versusthe BP concentration. To calculate the concentration, FL-P dispersionswith different volumes were filtered on an AAO membrane with 20 nm poresize, and then the mass difference was compared before and afterfiltration.

FIGS. 6A-B. AFM derived flake thickness distributions. Thicknesshistogram (A) and area histogram (B) of FL-P nanosheets prepared inSDS-water. The layer thickness was defined as ˜1 nm from the thicknesshistogram with the smaller scale (brown).

FIGS. 7A-C. Optical properties of FL-P aqueous dispersions. A, Opticalabsorbance spectrum of FL-P and a higher resolution plot in the rangebetween 900 nm and 1250 nm (inset). B, PL spectra of FL-P aqueousdispersions. Three peaks corresponding to monolayer, bilayer, andthree-plus layer phosphorene are observed at ˜907 nm, ˜1215 nm, and˜1428 nm, respectively. C, PL spectra of FL-P dispersions (red) andO2-exposed FL-P dispersions (blue), and XPS spectra of O2-exposed FL-Pdispersions.

FIGS. 8A-D. Digital images and photoluminescence (PL) spectra ofexfoliated, monolayer phosphorene. A, Optical image of a monolayerphosphorene flake (blue outline), passivated from ambient oxidation with˜2.6 nm of ˜50° C. AlOx.2 B, PL spectra of the phosphorene flake in (a),excited with a linearly polarized, 532 nm laser at ˜15 μW (100×, 0.9 NAobjective, 10 s acquisition in air). The phosphorene monolayer emits at924 nm (˜1.34 eV), near the expected wavelength, whereas emissions fromthe SiO2 substrate (purple) are absent. Additional emissions in thevisible are present (761 nm, 823 nm, and 867 nm, respectively),potentially related to oxidized BP regions. C, Optical image of a secondphosphorene region (blue outline), passivated like (a). D, PL spectra ofthe flake in (c) after successive spectral acquisitions with a 532 nmlaser at ˜15 μW (100×, 0.9 NA objective, 20 s acquisition in air). Whilethe phosphorene monolayer has its excitonic emission at ˜918 nm (˜1.35eV), it possesses a small oxide peak at ˜854 nm (˜1.45 eV). Withadditional measurements, the monolayer phosphorene emission decreases inintensity with the concurrent emergence of oxide-related bands (ca. 783nm and 827 nm, respectively), indicative of photooxidation.

FIGS. 9A-E. Digital images and PL spectra of bilayer phosphorene withthree-layer and four-layer regions. A, Optical image of a bilayerphosphorene flake (dark blue) attached to a thick BP region, passivatedby ˜10.0 nm AlO_(x) (˜2.5 nm at 50° C. ALD, ˜7.5 nm at 150° C. ALD). B,AFM height image of the flake in (a), showing bilayer, three-layer, andfour-layer phosphorene areas. C, Height profile along the contour in(b). Bilayer (˜1.3 nm), three-layer (˜1.7 nm), four-layer (˜2.2 nm)steps indicated (˜0.5 nm from instrumental factors, AlO_(x) overlayerhomogeneity, and adhesion). D, PL spectra of the bilayer phosphorene(blue spot, a), excited with a 532 nm laser at ˜150 μW (50×, 0.8 NAobjective, 5 s acquisition in air). The laser polarization was adjustedto maximize the bilayer emission relative to the ˜1143 nm (˜1.08 eV) Siemission (termed “parallel” orientation: □). Under parallel orientation,the bilayer phosphorene emits at 1279 nm (˜0.97 eV). To confirm theanisotropic optical properties of BP, the bilayer emission was dampenedfive-fold by rotating the laser polarization)90° (±20°) relative to theparallel orientation (termed “perpendicular” orientation: ⊥). E, PLspectrum from three-layer (green spot) and four-layer (magenta spot)phosphorene steps of (b), excited like (d). Spectrum taken from a singlearea on the bilayer phosphorene. While the bilayer phosphorene emissionpersists, emissions occur at 1488 nm (˜0.83 eV) and 1535 nm (˜0.81 eV)for three-layer and four-layer phosphorene, respectively.

FIGS. 10A-D. Digital images and polarized PL spectra of three-layer andfour-layer phosphorene. A, Optical image of a three-layer phosphoreneflake, passivated by ˜10.0 nm AlO_(x) (˜2.5 nm at 50° C. ALD, ˜7.5 nm at150° C. ALD). B, Polarization dependent PL spectra for the three-layerphosphorene in (a), ranging from minimum (“perpendicular”) to maximum(“parallel”) phosphorene emission (blue-dotted) at 1468 nm (˜0.84 eV).Polarization invariant Si emission (red-dotted) at ˜1137 nm (˜1.09 eV).The sample was excited with a 532 nm laser at ˜200 μW (50×, 0.8 NAobjective, 5 s acquisition in air). C, Optical image of a four-layerphosphorene flake, encapsulated like (a). D, Polarization dependent PLspectra for the four-layer phosphorene in (c), with maximum phosphoreneemission (blue-dotted, “parallel”) at ˜1522 nm (˜0.81 eV) and Siemission (red-dotted) at ˜1134 nm (˜1.09 eV). The sample was excited at˜250 μW with all other exposure conditions being the same as (b).

FIGS. 11A-D. PL spectra of passivated, five-layer phosphorene. A,Optical image of a five-layer phosphorene flake, passivated by ˜12.6 nmAlO_(x) (˜2.6 nm at 50° C. ALD, ˜10 nm at 150° C. ALD). B, AFM heightimage of the boxed region in (a). C, Height profile along the line in(b). Passivated phosphorene flake is ˜2.7 nm tall, indicative offive-layer phosphorene (˜0.5 nm additional, as aforementioned). D, PLspectrum of the phosphorene flake in (b), excited in the “parallel”orientation with a 532 nm laser at ˜150 μW (50×, 0.8 NA objective, 5 sacquisition in air). Passivated, five-layer phosphorene emits (blue) at1582 nm (˜0.78 eV), and the Si substrate emits (red) at 1129 nm (˜1.10eV), as expected.

FIGS. 12A-E. Digital images and solid-state PL spectra of BP nanosheetscast from solution. A, PL spectrum of FL-P aggregates on a 300 nmSiO₂/Si substrate. B, Optical image of FL-P aggregates on the substrate.C, PL mapping image displaying a peak position of ˜909 nm (˜1.36 eV).Overlaying (b) with (c) shows that the PL emission comes from theaggregated FL-P. D, PL spectra with respect to UV ozone exposure time.With increasing exposure, the PL emission intensity at ˜909 nm decreasesfrom monolayer phosphorene oxidation. Furthermore, an emission peakdevelops at ˜780 nm (˜1.59 eV), likely related to PO_(x) species.Finally, a SDS Raman band disappears with 60 min UV exposure. E,Emission intensities at ˜780 nm (green) and ˜909 nm (red) as a functionof UV ozone exposure time. The peak at ˜909 nm decreases monotonicallyas the FL-P is oxidized. The peak at ˜780 nm increases initially butultimately decays from extended chemical modification.

FIGS. 13A-C. Digital images and infrared, solid-state PL spectra ofsolution cast BP nanosheets. A, Optical image of unencapsulated FL-Paggregates on native oxide (SiO_(x)) on Si(100). B, Visible PL spectrumfor the aggregates in (a), exposed to a ˜350 μW, 532 nm laser (50×, 0.8NA objective, 15 s acquisition in air). Monolayer phosphorene emissionis present at 909 nm (˜1.36 eV), with an additional emission at ˜797 nm(˜1.56 eV). Spectrum agrees well with the FL-P aggregate emissions ofFIG. 12. C, Infrared PL spectra for the aggregates in (a). ControlSiO_(x)/Si(100) spectrum (purple) shows a weak Si emission (red) at˜1121 nm (˜1.11 eV). FL-P aggregates are broadband emitters, havingbilayer (1261 nm, ˜0.98 eV), three-layer (1459 nm, ˜0.85 eV), four-layer(1528 nm, ˜0.81 eV), and five-layer (1588 nm, ˜0.78 eV) phosphoreneemissions. A band tail from monolayer phosphorene (see (b)) also exists(peak at 1035 nm given for reference). Solution-cast phosphoreneemission energies are consistent with the mechanically exfoliated BPspectra of FIGS. 8-11. All peak emission wavelengths determined by Voigtfits to the baseline-corrected spectra.

FIGS. 14A-F. Digital images and graphs illustrating size sorting andelectrical properties of FL-P aqueous dispersions. A, Images of FL-Psolution before (left) and after (right) size sorting usingsedimentation-based density gradient ultracentrifugation (s-DGU). B,Histogram of flake areas for fractions f1 to f6. C, False-colored SEMimage of an FL-P nanosheet FET with higher magnification image of thechannel (inset). D, Output curves for a FL-P FET. E, Transfer curve fora FL-P FET plotted in linear scale (blue) and semi-log scale (green) forV_(DS) values of −1 V (open circles) and −25 mV (closed circles). F,Plot of I_(ON)/I_(OFF) versus drive current that compares this work topreviously reported BP FETs.

FIGS. 15A-C. Digital images showing area of size-sorted FL-P nanosheets.AFM images of fractions 1, 3, and 5 (FIGS. 15A, 15B, and 15C,respectively) after sedimentation-based density gradientultracentrifugation (Scale bars: 1 μm, Height: 10, 30, and 30 nm,respectively).

FIG. 16. FET measurements of FL-P nanosheets. The transfer curve of theFET article/device shown in FIG. 14E with the forward and reverse sweep.

FIGS. 17A-B. Histograms of electrical properties of FL-P FETs. Histogramof A, drive current, and B, I_(ON)/I_(OFF) ratio with logarithmic scale.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

As relates to certain non-limiting embodiments, this invention providesa scalable, high-yield, and environmentally benign method for preparingFL-P via ultrasonication in deoxygenated water. FIG. 1A depicts aprocedure for obtaining FL-P enriched dispersions and relatedcompositions. To minimize BP chemical degradation, deoxygenated water isprepared by purging deionized water with ultrahigh purity Ar gas in asealed container. Bulk BP crystals are then exfoliated with tipultrasonication in a vessel sealed in an Ar environment that containsdeoxygenated water with 2% w v⁻¹ sodium dodecylsulfate (SDS). For FL-Penrichment, as-prepared BP solutions undergo two steps ofsedimentation-based ultracentrifugation to isolate thin flakes withrelatively large lateral area. Initially, the as-exfoliated solution isdark brown (FIG. 1B, left), softening to light pale yellow aftersolution dilution and thick flake removal (FIG. 1B, right).

The chemical, structural, and optical properties of the resulting FL-Psolutions are ascertained through transmission electron microscopy(TEM), Raman spectroscopy, photoluminescence (PL) spectroscopy, andX-ray photoelectron spectroscopy (XPS). FIG. 1C shows a representative,low-magnification TEM image of a FL-P nanosheet, while thehigh-resolution TEM (HRTEM) image of FIG. 1D reveals the atomicstructure. Selected area electron diffraction (SAED) patterns (FIG. 1E)confirm that the FL-P nanosheets are crystalline and orthorhombic instructure. The HRTEM and SAED data further show no evidence ofstructural disorder or oxidation. In FIG. 1F, a liquid-phase Ramanspectrum from the FL-P dispersion shows the three representative BPRaman modes at ˜362 cm⁻¹ (A_(g) ¹), ˜439 cm⁻¹ (B_(2g)), and ˜466 cm⁻¹(A_(g) ²), with corresponding full-width at half-maximum (FWHM) valuesof 3.0, 3.3, and 5.1 cm⁻¹, respectively. Monolayer and few-layerphosphorene flakes have higher FWHM values than thicker (>5 nm) BP,making the FWHM broadening compared to precipitated, thicker BPdispersions indicative of FL-P enrichment in this process (FIGS. 2A-B).A visible (Si CCD) PL spectrum of the FL-P dispersion taken with 532 nmexcitation reveals an emission peak at ˜1.37 eV, consistent withprevious reports for monolayer phosphorene (FIG. 1G). Finally, thechemical quality of the FL-P flakes is assessed using XPS in FIG. 1H.XPS shows that FL-P exhibits the P2p_(3/2) and P2p_(1/2) doublet,characteristic of crystalline BP. Weak oxidized phosphorus (i.e.,PO_(x)) sub-bands are also observed at ˜136 eV in agreement withprevious measurements of electronic-grade BP. XPS taken on FL-P preparedwith other conventional surfactants (e.g., SDS, Pluronic F68 and sodiumcholate) display similar results (FIGS. 3A-C).

Although BP has been described as highly hydrophilic, the FL-Pdispersion is only stabilized with the incorporation of amphiphilicsurfactants in aqueous solution. While an aqueous BP dispersion preparedwith SDS results in a stable dark brown solution (FIG. 4A, left), BPdispersed in water without surfactants precipitates quickly (FIG. 4A,right). To clarify this apparent contradiction, the hydrophilicity ofthe BP surface was measured with contact angle measurements on a freshlycleaved flat BP crystal. FIG. 4B shows that, immediately followingexfoliation, the BP surface has an average contact angle of ˜57°,indicating that the hydrophilicity of BP is between graphene oxide(˜27°) and other 2D nanomaterials (˜90°) such as graphene and transitionmetal dichalcogenides. From the higher optical absorbance (FIG. 4C) andlower zeta potential value (FIG. 4D), the relatively hydrophobic,freshly exfoliated BP nanosheets are stabilized in aqueous solution withamphiphilic surfactants.

To investigate BP stability, LPE in both SDS-water and NMP was performedunder identical exfoliation and centrifugation conditions.Centrifugation in each case occurred in steps ranging from 500 r.p.m. to15,000 r.p.m. in order to compare the concentration of the resultingdispersions. FIGS. 4E and 4F show that the BP dispersion in NMPpossesses a lighter yellow color, indicative of a lower concentration,compared to BP in SDS-water. The actual concentration of BP in SDS-waterwas calculated from the optical absorbance at 660 nm, using the measuredextinction coefficient shown in FIG. 5 (the extinction coefficient thatwas used for BP in NMP was reported in the literature). This analysisconcluded that the concentration of BP in SDS-water is approximately anorder of magnitude higher than that of BP in NMP after centrifuging at15,000 r.p.m. (FIG. 4G). Atomic force microscopy (AFM) images of BPexfoliated in SDS-water (red) and NMP (green) followed by 7,500 r.p.m.centrifugation are shown in FIGS. 4H-I. The thickness histogram in FIG.4J reveals that FL-P nanosheets prepared in SDS-water have a tighterthickness distribution and thinner average thickness (4.5 nm compared to17.6 nm) than BP prepared in NMP. Additional thickness and areahistograms for FL-P prepared in SDS-water with smaller bins are providedin FIGS. 6A-B. These results illustrate the effectiveness of aqueoussurfactant solutions for producing thin FL-P nanosheets compared toorganic solvents.

Following FL-P composition/solution preparation, optical absorbancespectra were measured (FIG. 7A). A higher resolution optical absorbancespectrum shows two peaks between 900 nm and 1250 nm (FIG. 7A, inset),which is consistent with previous reports for monolayer and bilayerphosphorene. PL spectroscopy was also performed on the FL-P solutionusing an excitation wavelength of 532 nm and a Si CCD for emissionwavelengths up to 1000 nm and a N₂-cooled InGaAs array for emissionwavelengths between 1000 nm and 1600 nm. In FIG. 7B, the measured PLspectra were fit with Gaussian functions at peak positions of ˜907 nm(red area), ˜1215 nm (orange area), and ˜1428 nm (green area). Tocorrelate these peak positions with the number of phosphorene layers,micromechanically exfoliated BP flakes were prepared on a 300 nm SiO₂/Sisubstrate and then passivated with thin Al₂O₃ films. PL spectra andcorresponding optical microscopy images for flakes ranging in thicknessfrom one layer (1L) to five layers (5L) and more are shown in FIGS.8-11. Based on the PL peak positions measured for the micromechanicallyexfoliated BP flakes, the peaks at ˜907 nm, ˜1215 nm, and ˜1428 nm fromthe FL-P dispersion (FIG. 7B) are assigned to 1L, 2L, and 3L+phosphorene, respectively.

Additionally, solid-state PL spectra were measured on articlescomprising FL-P aggregates deposited onto a SiO₂/Si substrate using anexcitation wavelength of 532 nm. The measured PL spectra have a 1L peakposition at ˜909 nm, as shown in FIG. 12A. In FIG. 13, additional peakswere observed for 2L at ˜1261 nm and 3L+ at ˜1459 nm, ˜1528 nm, and˜1588 nm using a thermoelectrically cooled InGaAs array. The positionsof the PL peaks from FL-P are indicated in Table 1. The peak positiondifferences between the liquid-phase sample and the solid-state samplecan be attributed to the different dielectric screening from thesurrounding environment.

TABLE 1 Optical emissions for monolayer and few-layer phosphorenesheets. All emissions probed by solid-state PL spectroscopy. Errors arepropagated from the FWHM values of each phosphorene emission. Layernumber Isolation scheme Primary emission (eV) Monolayer Mechanicalexfoliation 1.34 ± 0.15 eV Bilayer Mechanical exfoliation 0.97 ± 0.16 eVThree-layer Mechanical exfoliation 0.85 ± 0.09 eV Four-layer Mechanicalexfoliation 0.82 ± 0.10 eV Five-layer Mechanical exfoliation 0.78 ± 0.06eV Monolayer Solution cast 1.38 ± 0.18 eV Bilayer Solution cast 0.98 ±0.29 eV Three-layer Solution cast 0.85 ± 0.03 eV Four-layer Solutioncast 0.81 ± 0.05 eV Five-layer Solution cast 0.78 ± 0.03 eV

To verify that the observed PL is not defect-mediated, FL-P aggregateswere exposed to UV ozone to intentionally introduce oxygen defects. Asthe UV ozone exposure time increases, the PL emission intensity at ˜909nm decreases and a PO_(x) defect-mediated emission peak emerges at ˜780nm (FIG. 12D). The emission intensities at ˜909 nm (red curve) and ˜780nm (green curve) are plotted as a function of UV ozone exposure time,with the 909 nm peak monotonically decreasing as the FL-P is oxidized,and the 780 nm peak initially increasing in intensity due to increaseddefect concentration and ultimately decaying due to the completedestruction of the material (FIG. 12E). This observation correspondsclosely to the time-evolution of PL with respect to photo-oxidation timefor micromechanically exfoliated BP (FIG. 8D). Furthermore, PL spectrawere measured under the same excitation conditions on the FL-Pdispersion before (red area) and after oxidation introduced by O₂ gasbubbling (blue area). Following oxidation, the FL-P shows significantlydecreases in PL intensity (FIG. 7C, left) in addition to substantialincreases in the PO_(x) XPS peak (FIG. 7C, right). Consequently, the PLpeaks observed in FIG. 7B can be attributed to 1L, 2L, and 3L+phosphorene, as opposed to defects introduced by solution processing.

To explore the electrical properties of individual FL-P nanosheets,field-effect transistors (FETs) were fabricated by electron-beamlithography (EBL). Prior to FET device fabrication, lateral size sortingof the FL-P solution was performed to enrich large FL-P nanosheets thatcan bridge the 200 nm electrode gap. Lateral size sorting of the FL-Psolution was achieved by sedimentation-based density gradientultracentrifugation (s-DGU). By using s-DGU, the average flake area wastuned by over an order of magnitude (FIG. 14A). Each fraction (f1 to f6from the top to the bottom of the centrifuge tube) was collected anddeposited onto Si substrates for AFM measurements. The AFM images of f1,f3, and f5 clearly show the size differences between each fraction (FIG.15). From the AFM measurements, the average areas of f3 to f6 were foundto be appropriate for FET fabrication (FIG. 14B).

To prepare arrays of the FL-P nanosheets, fractions f3 to f6 werecollected on anodic aluminum oxide (AAO) membranes by vacuum filtration,rinsed with deoxygenated water to remove excess surfactant, andtransferred onto degenerately doped Si substrates coated with 20 nm ofatomic layer deposition (ALD) Al₂O₃ or HfO₂. Following the FL-Ptransfer, Au/Ni electrodes of 10 μm width and 200 nm length werepatterned using EBL. A false-colored scanning electron microscopy (SEM)image (FIG. 14C) shows several FL-P nanosheets (green) connected inparallel between two electrodes (yellow). FET output and transfer curves(FIGS. 14D-E) reveal ambipolar behavior for FL-P with a currentmodulation (I_(ON)/I_(OFF) ratio) of ˜5×10³ and maximum drive current(I_(DS)) of ˜130 μm⁻¹ at V_(DS)=1 V (the transfer curve for the forwardand reverse gate voltage sweep is shown in FIG. 16). Histograms of themaximum drive current and current modulation from several FL-P FETs areshown in FIG. 17. For comparison, I_(ON)/I_(OFF) ratio and maximumI_(DS) values for BP FETs from the recent literature (bothmicromechanically exfoliated and organic solvent exfoliated) are plottedin FIG. 14F (Ref 3: Xia, F., Wang, H. & Jia, Y. Rediscovering blackphosphorus as an anisotropic layered material for optoelectronics andelectronics. Nature Comm. 5, 4458 (2014); Ref. 6: Kang, J. et al.Solvent exfoliation of electronic-grade, two-dimensional blackphosphorus. ACS Nano 9, 3596-3604 (2015); Ref. 10: Wood, J. D. et al.Effective passivation of exfoliated black phosphorus transistors againstambient degradation. Nano Lett. 14, 6964-6970 (2014); Ref 30: Du, Y.,Liu, H., Deng, Y. & Ye, P.D. Device perspective for black phosphorusfield-effect transistors: contact resistance, ambipolar behavior, andscaling. ACS Nano 8, 10035-10042 (2014); Ref. 31: Miao, J., Zhang, S.,Cai, L., Scherr, M. & Wang, C. Ultrashort channel length blackphosphorus field-effect transistors. ACS Nano 9, 9236-9243 (2015); andRef. 32: Wang, H. et al. Black phosphorus radio-frequency transistors.Nano Lett. 14, 6424-6429 (2014).)

Results from the FL-P device from FIG. 14E are also shown as a yellowstar on this plot. Since the upper right hand corner of this plotrepresents an optimal combined performance from the perspective ofmaximum drive current and current modulation, it is evident thataqueous-exfoliated FL-P compares favorably with competing BP exfoliationmethods from the perspectives of electronic properties and utility forelectronic devices.

More generally, relating to various embodiments of this invention, avariety of surface active components are effective at dispersing boththe three-dimensional starting materials and the exfoliatedtwo-dimensional planar nanomaterials. Without limitation, one or moresurface active components can include one or more bile salts and/oralkali salts of linear alkyl sulfonates. Bile salts can be more broadlydescribed as a group of molecularly rigid and planar amphiphiles withone or more charged groups opposing a hydrophobic face. Examples of bilesalts include salts (e.g., sodium or potassium salts) of conjugated orunconjugated cholates and cholate derivatives including deoxycholates,chenodeoxycholates, taurodeoxycholates, glycochenodeoxycholates,ursodeoxycholates, and glycoursodeoxycholates.

Useful alkyl sulfates include butane not limited to sodium hexylsulfate, sodium octyl sulfate, sodium decyl sulfate, sodium undecylsulfate, sodium dodecyl sulfate, and lithium dodecyl sulfate. Otheruseful surface active components include non-ionic triblock copolymersof oxyethylene and oxypropylene (or poly(ethylene oxide) andpoly(propylene oxide), respectively), known under the Pluronic tradename), which can be represented by the general formulaHO(C₂H₄O)_(a)(C₃H₆O)_(b)(C₂H₄O)_(a)H, wherein a and b are integers inthe range of about 10 to about 300; and non ionic tetrafunctional blockcopolymers of oxyethylene and oxypropylene known under the Tetronictrade name. Various other amphiphilic surface active components usefulin conjunction with this invention would be well-known to those skilledin the art, limited only by functional precluding capacity to exfoliatephosphorus starting material and disperse resulting few-layerphosphorene.

Generally, density gradient ultracentrifugation uses a fluid medium witha predefined variation in its density as a function of position within acentrifuge tube or compartment (i.e., a density gradient). Fluid mediauseful with the present teachings are limited only by nanomaterialaggregation therein to an extent precluding at least partial separation.Accordingly, aqueous and non-aqueous fluids can be used in conjunctionwith any substance soluble or dispersible therein, over a range ofconcentrations, so as to provide the medium a density gradient for usein the separation techniques described herein. Such substances can beionic or non-ionic, non-limiting examples of which include inorganicsalts and alcohols, respectively. Such a medium can include a range ofaqueous iodixanol concentrations and the corresponding gradient ofconcentration densities. As understood by those skilled in the art,aqueous iodixanol is a common, widely used non-ionic density gradientmedium. However, other media can be used in methods of the presentteachings, as would be understood by those skilled in the art.

More generally, any material or compound stable, soluble or dispersiblein a fluid or solvent of choice can be used as a density gradientmedium. A range of densities can be formed by dissolving such a materialor compound in the fluid at different concentrations, and a densitygradient can be formed, for instance, in a centrifuge tube orcompartment. More practically, with regard to choice of medium, thetwo-dimensional nanomaterials in composition with the surface activecomponents should be soluble, stable or dispersible within thefluids/solvent or resulting density gradient. Likewise, from a practicalperspective, the maximum density of the gradient medium, as determinedby the solubility limit of such a material or compound in the solvent orfluid of choice, should be at least as large as the buoyant density ofthe nanomaterial-surface active component complexes for a particularmedium. Accordingly, any aqueous density gradient medium can be usedprovided that the nanomaterials are stable; that is, do not aggregate toan extent precluding useful separation. Alternatives to iodixanolinclude inorganic salts (such as CsCl, Cs₂SO₄, KBr, etc.), polyhydricalcohols (such as sucrose, glycerol, sorbitol, etc.), polysaccharides(such as polysucrose, dextrans, etc.), other iodinated compounds inaddition to iodixanol (such as diatrizoate, nycodenz, etc.), andcolloidal materials (such as Percoll®).

EXAMPLES OF THE INVENTION

The following non-limiting examples and data illustrate various aspectsand features relating to the methods, materials, compositions, articlesand/or devices of the present invention, including the preparationand/or separation of few-layer phosphorene as are available through themethodologies described herein. In comparison with the prior art, thepresent methods, compositions and articles/devices provide results anddata which are surprising, unexpected and contrary thereto. While theutility of this invention is illustrated through the use of severalphosphorene materials, compositions, separation fractions, media andsurface active agents which can be used therewith, it will be understoodby those skilled in the art that comparable results are obtainable withvarious other phosphorene materials, compositions, fractions thereof,media and/or surface active agents, as are commensurate with the scopeof this invention.

Example 1

FL-P dispersion preparation. BP crystals were purchased fromSmart-Elements and stored in a dark Ar glovebox prior to use. Deionizedwater with 2% w v⁻¹ surfactant was purged with ultrahigh purity grade Arfor at least 1 hr to remove dissolved oxygen. A customized tipultrasonicator setup was prepared by perforating the plastic lid of a 50mL conical tube with a 0.125 inch sonicator tip. The interface betweenthe tip and the lid was sealed with PDMS several times to blockpotential diffusion pathways of ambient O₂ and H₂O. The deoxygenatedwater and BP were placed in this sealed conical tube with an initialconcentration of 1 mg mL⁻¹ under Ar atmosphere with less than 10%relative humidity. Additionally, Parafilm and Teflon tapes were used tofurther seal the vessel from ambient exposure. The sealed container wasconnected to the ultrasonicator (Fisher Scientific model 500 sonicdismembrator) in ambient conditions, and then BP crystals wereexfoliated by ultrasonication. The resulting solution was centrifuged at7,500 r.p.m. for 2 hrs at 15° C. to enrich FL-P nanosheets (Avanti J-26XP, Beckman Coulter). Following centrifugation, the supernatant wascollected and then ultracentrifuged at 14,000 r.p.m. for 2 hrs at 22° C.using a SW32Ti rotor (Optima L-80 XP, Beckman Coulter) beforeredispersing in deoxygenated water.

Example 2

Size selection of FL-P. 3 mL of as-prepared FL-P dispersion wascarefully placed on top of a 10 mL linear density gradient (1.05 to 1.10g cm⁻³) formed using deoxygenated iodixanol that was loaded with 2% wv⁻¹ SDS. Ultracentrifugation was then performed at 10,000 r.p.m. for 1hr at 22° C. using a SW41 Ti rotor (Beckman-Coulter). Followingultracentrifugation, fractionation was achieved using a piston gradientfractionator (BioComp Instruments).

Example 3

Contact angle measurements. Water droplet contact angles on BP crystalswere measured by a contact angle goniometer. Flat BP crystals werepurchased from HQ Graphene. Fresh BP surfaces were prepared bymicromechanical exfoliation with water droplets placed on the surfacewithin ˜30 sec. A high-resolution camera attached on the goniometercaptured images from which the contact angle was determined.

Example 4

Zeta potential measurement. Zeta potential measurements were carried outusing a Zetasizer Nano ZS (Malvern Instruments) with clear disposablezeta cells. A He—Ne laser source with a wavelength of 633 nm and amaximum power of 5 mW was used for the measurements. The reportedresults are averages from three independent measurements at 25° C.

Example 5

Atomic force microscopy (AFM). AFM images were acquired in tapping modeusing an Asylum Cypher AFM with Si cantilevers (˜290 kHz resonantfrequency). As-prepared solutions were deposited onto Si substrates,rinsed with deoxygenated water to remove surfactant, and dried on a hotplate at 80° C. for 10 min in a flowing Ar environment. Prior todeposition, Si substrates were rinsed with acetone, methanol, anddeionized water and immersed in diluted (3-aminopropyl)-triethoxysilane(APTES) solution to promote adhesion. After BP deposition on the Sisubstrate, an environmental cell was assembled in a flowing Arenvironment and attached to a Cypher ES scanner under a laminar flow ofultrahigh purity grade N₂. Images were taken in the repulsive phaseregime at a scanning rate of ˜0.4 Hz using a minimum of 1024 samples perline. During scanning, N₂ was continuously flowed through theenvironmental cell under optical microscopy light illumination.

Example 6

Transmission electron microscopy (TEM). A BP solution droplet wasdeposited on a holey carbon TEM grid (Ted-Pella) and dried with N₂. TheTEM grid was assembled with a TEM sample holder after fewer than 5 minof exposure to ambient air. The TEM images were taken with a JEOLJEM-2100 at an accelerating voltage of 200 keV with a TEM columnpressure of ˜10⁻⁷ Torr.

Example 7

Raman spectroscopy. Raman spectra of the solutions were obtained using aHoriba LabRAM HR Evolution with an excitation wavelength of 532 nm. Aclear quartz cuvette with 10 mm transmitted path length was used for themeasurement. Data were collected for 120 sec at ˜50 mW using an angledcuvette holder for the solution samples.

Example 8

X-ray photoelectron spectroscopy (XPS). XPS measurements were performedusing a high vacuum Thermo Scientific ESCALAB 250 Xi XPS system at abase pressure of ˜1×10⁻⁹ Ton. The XPS data had a binding energyresolution of ˜0.1 eV using a monochromated Al Kα X-ray source at˜1486.7 eV (˜400 μm spot size). All core-level spectra were the averageof five scans taken at a 100 ms dwell time using a pass energy of 15 eV.When using charge compensation, all core levels were charge-corrected toadventitious carbon at ˜284.8 eV. Using the software suite Avantage(Thermo Scientific), all subpeaks were determined in a manner detailedin the literature. (Kang, J. et al. Solvent exfoliation ofelectronic-grade, two-dimensional black phosphorus. ACS Nano 9,3596-3604 (2015).) The p core level spectra for phosphorus and siliconwere fit with doublets.

Example 9

Photoluminescence (PL) spectroscopy. PL spectra for the solution sampleswere obtained using a Horiba Fluorolog-3 spectrofluorometer. Data weremeasured in a quartz cuvette for 3 sec. A liquid N₂ cooled InGaAs arraywas used for the spectra at wavelengths longer than 1000 nm. PL spectrafor the solid-state samples on 300 nm SiO₂/Si substrates were collectedusing a Horiba Xplora Raman/PL system with an excitation wavelength of532 nm. Data were collected for 120 sec using a 100× objective for pointspectra and a 20 × objective for mapping. This setup was used for thedata in FIGS. 7 and 8. For the infrared PL measurements in FIGS. 9-11,and 13, a spectrometer (Princeton Aston) with a NIRvana InGaAsthermoelectrically cooled array at an excitation wavelength of 532 nmwas employed. Data were collected for 5 sec using a 100× near-infraredobjective. All PL spectral fits were performed with spline baselines andVoigt (Gaussian-Lorentzian) functions. Spectral fits disregarded anylineshape changes due to differences in grating efficiency at off-blazewavelengths.

Example 10

Optical absorbance spectroscopy. Optical absorbance spectra wereobtained using a Cary 5000 spectrophotometer (Agilent Technologies). Aquartz cuvette with 10 mm path length was used for the measurements. Thebaseline from the aqueous surfactant solution was subtracted from thespectra.

Example 11

FL-P nanosheets transfer by PDMS stamping. The few-layer phosphorenenanosheets were collected on anodic aluminum oxide (AAO) membranes with100 nm pore size by vacuum filtration. Following vacuum filtration, thenanosheets on the membrane were rinsed with ˜300 mL of deoxygenatedwater to remove the surfactants. The nanosheets on the membrane werethen transferred onto the target substrates using PDMS stamping.

Example 12

Field-effect transistor fabrication and measurement. Field-effecttransistors (FETs) were fabricated using electron beam lithography todefine 200 nm long, 10 μm wide electrodes (5 nm Ni and 30 nm Au). BPFETs were measured in a Lakeshore CRX 4K under a base pressure of lessthan ˜5×10⁻⁴ Torr at room temperature. Two Keithley sourcemeter 2400units were used to measure device performance. Equation 1 was used tomeasure carrier mobility:

$\begin{matrix}{\mu_{eff} = \frac{{Lg}_{d}}{{WC}_{ox}V_{DS}}} & (1)\end{matrix}$where μ_(eff) is the field-effect mobility, L is the channel length(obtained from optical micrographs), g_(d) is the transconductance, W isthe channel width (obtained from optical micrographs), C_(ox) is theoxide capacitance (measured values of 1.02×10⁻² and 2.00×10⁻³ F·cm⁻²were used for 20 nm thick ALD HfO₂ and Al₂O₃, respectively), and V_(DS)is the applied source-drain bias.

As demonstrated, effective exfoliation of FL-P nanosheets has beenachieved by ultrasonication in deoxygenated water stabilized withsurfactants. This method results in stable, highly concentratedfew-layer phosphorene, with distinct advantages compared to exfoliationin organic solvents. Comprehensive microscopic and spectroscopicanalysis shows that individual FL-P nanosheets possess propertiescomparable to micromechanically exfoliated BP flakes without chemicaldegradation following aqueous processing. For example, PL measurementsdemonstrate that FL-P aqueous dispersions show strong visible andnear-infrared emission that is characteristic of 1L, 2L, and 3L+phosphorene. FL-P aqueous dispersions are also amenable to further sizesorting by s-DGU, which allows for the enrichment of flakes with largelateral areas suitable for FET fabrication. The resulting FETs confirmthat aqueous-exfoliated FL-P can be incorporated into electronic deviceswith performance metrics competitive with the best BP transistors todate.

We claim:
 1. A method of preparing few-layer phosphorene, said methodcomprising: providing a composition comprising a black phosphorusstarting material, at least one amphiphilic surface active component anddeoxygenated water; sonicating said composition to provide adeoxygenated aqueous medium comprising exfoliated phosphorousnanomaterials; and centrifuging said aqueous medium to provide asupernatant component comprising a polydisperse population of planarfew-layer phosphorene nanomaterials comprising mono-, bi-, and n-layerfew-layer phosphorene nanosheets, wherein n is an integer selected from3- about 6, wherein said surface active component is selected fromalkali metal alkylsulfates and combinations thereof, or from non-ionictriblock oxyalkylene copolymers and combinations thereof.
 2. The methodof claim 1 wherein said surface active component is sodiumdodecylsulfate.
 3. The method of claim 1 wherein said surface activecomponent is a copolymer of oxyethylene and oxypropylene.
 4. The methodof claim 1 comprising separating said few-layer phosphorenenanomaterials into two or more separation fractions comprising asubpopulation of few-layer phosphorene nanosheets comprising a surfacearea, each said fraction comprising monolayer phosphorene nanosheets,bilayer phosphorene nanosheets and trilayer phosphorene nanosheets, orcombinations thereof.
 5. The method of claim 4 wherein said separationcomprises centrifuging said few-layer phosphorene nanomaterials in adeoxygenated fluid medium comprising a density gradient for at least oneof a time and at a rotational rate sufficient to provide said separationfractions.
 6. The method of claim 5 wherein said fluid medium comprisesdeoxygenated aqueous iodixanol.
 7. The method of claim 6 wherein saidfluid medium comprises a plurality of deoxygenated aqueous iodixanolconcentrations, said density gradient comprising a range ofconcentration densities.
 8. The method of claim 7 wherein a saidfew-layer phosphorene fraction is within said density gradient at asubstantially invariant point during centrifugation, said invariantpoint comprising a density approximating the buoyant density of a saidfew-layer phosphorene fraction.
 9. The method of claim 4 wherein atleast one said separation fraction is isolated from said fluid mediumand deposited on a substrate.
 10. The method of claim 9 wherein saidsubstrate and deposited few-layer phosphorene are incorporated into adevice selected from electronic and optoelectronic devices.
 11. A methodof preparing few-layer phosphorene, said method comprising: providing acomposition comprising a black phosphorus starting material, at leastone amphiphilic surface active component and deoxygenated water;sonicating said composition to provide a deoxygenated aqueous mediumcomprising exfoliated phosphorous nanomaterials; centrifuging saidaqueous medium to provide a supernatant component comprising apolydisperse population of planar few-layer phosphorene nanomaterialscomprising mono-, bi-, and n-layer few-layer phosphorene nanosheets,wherein n is an integer selected from 3- about 6; and separating saidfew-layer nanomaterials into two or more separation fractions comprisinga subpopulation of few-layer phosphorene nanosheets comprising a surfacearea, each said fraction comprising monolayer phosphorene nanosheets,bilayer phosphorene nanosheets and trilayer phosphorene nanosheets, orcombinations thereof, said separation comprising centrifuging saidfew-layer phosphorene nanomaterials in a deoxygenated fluid mediumcomprising a density gradient for at least one of a time and at arotational rate sufficient to provide said separation fractions, whereinsaid fluid medium comprises deoxygenated aqueous iodixanol.
 12. Themethod of claim 11 wherein said surface active component is selectedfrom alkali metal alkylsulfates and combinations thereof.
 13. The methodof claim 11 wherein said surface active component is selected from bilesalts and combinations thereof.
 14. The method of claim 11 wherein saidsurface active component is selected from non-ionic triblock oxyalkylenecopolymers and combinations thereof.
 15. The method of claim 11 whereinsaid fluid medium comprises a plurality of deoxygenated aqueousiodixanol concentrations, said density gradient comprising a range ofconcentration densities.
 16. The method of claim 11 wherein at least onesaid separation fraction is isolated from said fluid mediums depositedon a substrate and incorporated into a device selected from electronicand optoelectronic devices.